Eddy-current braking systems are used in a range of applications to provide non-contact braking and offer a significant advantage over conventional friction brakes as there is no frictional contact between the braking surfaces.
Eddy-current brakes function on the principle that when a conductor moves through a magnetic field (or vice-versa) the relative motion induces circulating ‘eddies’ of electric current in the conductor. The current eddies in turn induce magnetic fields that oppose the effect of the applied magnetic field. Eddy-current brakes thus utilise the opposing magnetic fields to act as a brake on movement of the conductor in the magnetic field, or vice versa. The strength of the eddy current magnetic field, and therefore the opposing force is dependant on a number of factors including the:                strength of the applied magnetic field;        magnetic flux through the conductor;        geometrical dimensions of the conductor and magnetic field, e.g. size, physical separation;        electrical conductivity of the conductor; and        relative velocity between the conductor and magnetic field.        
A variable braking force is thus achieved by varying any one or more of the above parameters.
To aid clarity and avoid prolixity, reference herein is made with respect to applications requiring a braking/retarding torque for rotating members and more particularly to an auto-belay system for which the present invention has particular application. However, reference herein to an auto-belay system should not be seen to be limiting as it will be appreciated by one skilled in the art that there are innumerable applications for eddy-current braking systems.
The speed of rotation (angular velocity) of the rotor with respect to the magnetic field will herein be referred to as the “rotation speed” or where convenient shortened to “speed”.
Rotary plate-type eddy-current braking systems typically use a paramagnetic conductive disc that is configured to rotate in a plane orthogonal to a magnetic field applied by magnets positioned on one or both sides of the disc. The eddy currents, and corresponding magnetic field, are generated when the disc is rotated relative to the magnetic field. A braking torque is thereby applied to the rotating disc. A higher relative velocity between the conductor and magnets will result in a higher braking torque thereby potentially limiting the rotation speed.
The braking torque is linearly proportional to the speed only until a threshold ‘characteristic speed’ is reached. Above this characteristic speed the braking torque response to speed becomes non-linear and peaks before beginning to reduce with further speed increases. This characteristic is illustrated in FIG. 1 which shows an approximate plot of braking torque against rotation speed for a typical disc-type eddy-current braking system. The characteristic speed is dependant on the resistivity of the disc which is dependant on the temperature, materials, magnetic permeability, and construction of the disc.
The braking torque of a typical eddy current disc system operating below the characteristic speed is determined approximately by the following relationship:T∝AdB2R2ω
Where the braking torque T is proportional to:
A—the conductor surface area intersecting the magnetic field;
B2—applied magnetic field strength—squared;
d—thickness of the disc;
R2—the radius (distance) from the axis of rotation to the conductor in the magnetic field—squared;
ω—rotational speed.
A non-linear response of braking torque may thus be achieved by varying the magnetic field strength B and/or the distance to the center of rotation R.
The magnetic field can be supplied by permanent magnets and/or electromagnets. The strength of the magnetic field is dependant on the magnetic field intensity and the configuration of the magnetic circuit, i.e. the materials used and spatial positions of the components in the system.
For permanent magnet systems, variation of the magnetic circuit (e.g. variation in A, d or R) is the most effective way to alter the braking torque. Typical eddy-current brake systems thus position the magnets toward the periphery of the disc to maximise R.
Common magnetic circuit configurations utilise permanent magnets positioned on one or both sides of the disc with steel backing behind each magnet. The steel plates are provided to enhance the magnetic field strength while providing structural support for the magnets.
An auto-belay device is used in climbing, abseiling and the like to control the descent rate of the climber. The auto-belay also automatically retracts line when the climber is ascending to maintain line tension thus avoiding slack occurring in the line.
Existing auto-belay systems typically use a friction-brake or hydraulic dampening mechanism to control the descent rate. Friction-brakes clearly have disadvantages compared with eddy-current brakes as the frictional contact involves substantial heat generation, wear and corresponding safety problems. Hydraulic dampening mechanisms are expensive and vulnerable to leaks, pressure and calibration problems.
An ideal auto-belay system would provide a constant or controllable descent rate with minimal friction and corresponding wear while also providing sufficient braking force in a small compact device.
The prior art is replete with various eddy-current braking systems. However, none of the prior art systems appear suitable for application in an auto-belay or other applications where a constant speed of rotation is required where the torque applied may vary.
Typical prior art plate-type braking systems use various magnetic circuit configurations and have attendant pros and cons. Examples of typical prior art devices are described below.
One prior art plate-type eddy-current braking device is described in U.S. Pat. No. 4,567,963 by Sugimoto and comprises a conductive disc coupled to a rotor via an overdrive gear arrangement to rotate the disc at a proportionally greater rotational speed than the rotor. The rotor includes a spool from which a line is dispensed. A series of permanent magnets are attached to an iron plate extending parallel to the disc's plane of rotation and spaced radially with respect to the axis of rotation. These magnets produce eddy-currents in the disc during rotation and, axiomatically, a corresponding magnetic field and braking effect. The Sugimoto system also includes radiator fins to assist in dissipation of the heat generated by the eddy-currents in the disc. The rotation of the rotor is retarded with an increasing force as the rotational speed (ω) increases. The overdrive arrangement provides an increased retardant force compared to a disc directly coupled to the rotor and thus, in the Sugimoto device, the aforementioned torque relationship would be similar to T∝AdB2R2kω where k is the overdrive gear ratio.
While the Sugimoto device may be more effective than simple plate-type systems it cannot be adjusted to vary the braking force applied and relies on an overdrive mechanism to improve braking force i.e. by increasing the relative speed between disc and magnetic field. The overdrive mechanism adds to cost, complexity, size, wear, increased heat generation and possibility of failure.
Furthermore, the speed of rotation of the rotor will still vary with the applied torque.
The Sugimoto device has provided a larger braking effect relative to smaller devices by varying the rotational speed. However, the gear mechanism constrains the limits of size and thus the degree of miniaturisation possible. The Sugimoto device is thus undesirable for auto-belays which require a compact device with safe, reliable operation during frequent, and/or prolonged use.
A similar device to that of Sugimoto is described in U.S. Pat. No. 5,342,000 by Berges et al. Berges et al. describe a plate-type eddy-current braking system with a centrifugal clutch that engages the eddy-current braking system when the rotor reaches a sufficient rotational speed.
It should be noted that neither the Sugimoto nor Berges et al devices can be adjusted to control the braking effect without disassembling and changing the overdrive gear ratios or magnet strength. Thus, these devices prove inconvenient in applications that need to accommodate different applied torques.
Attempts have been made at providing variable braking systems and exemplary devices are described in U.S. Pat. No. 4,612,469 by Muramatsu, EP 1,480,320 by Imanishi et al., U.S. Pat. No. 3,721,394 by Reiser and U.S. Pat. No. 6,460,828 by Gersemsky et al.
The Muramatsu device has a rotating disc with a manually adjustable position with respect to a magnet array, thus providing a means in which to vary the area (A) of magnetic field intercepted by the disc. The Muramatsu device may be adjustable to vary the braking effect and the maximum braking torque achievable but is still constrained by the size of the disc and strength of magnets, thus proving inconvenient where a smaller size is advantageous, e.g. for auto-belay devices. Furthermore, the Muramatsu device must be varied manually.
The device described by Imanashi et al works on a similar principle to that described by Muramatsu. However, instead of varying the disc area intersected by the magnetic field, the Imanashi et al system uses a magnet array attached to a linear drive to move the array axially away or toward the disc to respectively reduce or increase the separation and the magnetic field flux the disc intersects. As with the Muramatsu device, the braking effect of the Imanashi et al. cannot be automatically adjusted to accommodate different applied torques.
An automatic version of the Imanashi et al. device is described in U.S. Pat. No. 3,721,394 by Reiser and positions a line spool coupled to a conductive disc above a magnet array with a spring therebetween. As the line is unwound from the spool, the weight on the spring reduces and the spring extends, increasing the spacing between the disc and magnet and thereby decreasing the braking effect as the line is unwound. The spring is calibrated so that the speed of rotation of the spool remains constant as the line is unwound. The Reiser system is reliant on a static supporting arrangement and varying weight change in the spool and is thus unsuitable for an auto-belay device. Furthermore, the braking effect of the Reiser device varies only with rotation speed and magnetic field strength and not applied torque.
A brake for a hoist is described in U.S. Pat. No. 6,460,828 by Gersemsky et al. and uses a magnetic circuit that varies the position of a magnet with respect to a rotating conductive disc. The magnet is attached to a free end of a pivoting arm with a spring attached to the free end and to a static point adjacent the disc. As the disc rotates, the eddy-currents induced provide a braking effect on the disc to inhibit rotation. A reactive force is applied to the magnet by the braking effect to pivot the arm to move the magnet radially outward to increase braking torque. The spring will compress and oppose this reactive force thereby providing a braking effect on the disc. Reverse rotation of the disc will result in an opposing reactive force that, will force the magnet in an opposite direction, the spring then extending. and opposing the reactive force to apply the braking effect. Thus, the Gersemsky et al. system provides a sufficient braking effect regardless of the direction of rotation of the disc. The radial movement of the magnet also increases braking effect as a result of increasing relative velocity.
The Gersemsky et al. system, while fulfilling its purpose, is limited in adaptability as the braking torque applied is dependant on only the relative velocity (proportional to speed of rotation and radius to axis of rotation) of the magnets. Furthermore, auto-belay devices typically require braking in only one direction and thus universal braking devices such as the Gersemsky et al, system may be unsuitable.
It would thus be advantageous to provide an eddy-current braking mechanism that is capable of limiting the speed of rotation of a rotor over a wide range of applied loads or torques.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein; this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.